The detection and interdiction of illicitly trafficked Special Nuclear Material (SNM) is very important in the ongoing anti-terrorist activities undertaken by homeland security agencies. United States Patent Application No. 2005/0105665 by Lee Grodzins and Peter Rothschild for a system of detection of neutrons and sources of radioactive material, published May 19, 2005, provides the following state of technology information: “There is a need to find sources of radiation and other nuclear material that are clandestinely transported across national boundaries. The sources of clandestine nuclear material may be in the form of ‘dirty bombs’ (e.g., a conventional explosive combined with radioactive nuclides designed to spread radioactive contamination upon detonation), fissile material, and other neutron and radiation emitting sources that may present a hazard to the public. During recent years, the United States government has placed mobile vehicles at strategic areas with gamma ray detectors dedicated to the task of finding fissile material. Atomic explosives may be made from 235U (Uranium-235), a rare, naturally occurring, isotope of uranium that lives almost 109 years, or 239PU (Plutonium-239), a reactor-made isotope that lives more than 104 years. 235U decays with the emission of gamma ray photons (also referred to as ‘gammas’), principally at 185.6 keV and 205.3 keV. 239PU emits a number of gamma rays when it decays, the principal ones being at 375 keV and 413.7 keV. These gamma rays are unique signatures for the respective isotopes. But fissile material invariably contains other radioactive isotopes besides those essential for nuclear explosives. For example, weapons grade uranium may contain as little as 20% 235U; the rest of the uranium consists of other isotopes. The other uranium and plutonium isotopes reveal their presence by gamma rays emitted by their daughters. For example, a daughter of 238U emits a high energy gamma ray at 1,001 keV; a daughter of 232U, an isotope present in fissile material made in the former USSR, emits a very penetrating gamma ray at 2,614 keV; and a daughter of 241Pu emits gamma rays of 662.4 keV and 722.5 keV.”
U.S. Pat. No. 4,201,912 issued May 6, 1980 to Michael L. Evans et al and assigned to the United States of America as represented by the United States Department of Energy, provides the following state of technology information: “A device for detecting fissionable material such as uranium in low concentrations by interrogating with photoneutrons at energy levels below 500 keV, and typically about 26 keV. Induced fast neutrons having energies above 500 keV by the interrogated fissionable material are detected by a liquid scintillator or recoil proportional counter which is sensitive to the induced fast neutrons. Since the induced fast neutrons are proportional to the concentration of fissionable material, detection of induced fast neutrons indicates concentration of the fissionable material.”
U.S. Pat. No. 3,456,113 issued Jul. 15, 1969 to G. Robert Keepin and assigned to the United States of America as represented by the United States Atomic Energy Commission, provides the following state of technology information: “An apparatus and method of detecting, identifying and quantitatively analyzing the individual isotopes in unknown mixtures of fissionable materials. A neutron source irradiates the unknown mixture and the kinetic behavior of the delayed neutron activity from the system is analyzed with a neutron detector and time analyzer. From the known delayed neutron response of the individual fission species it is possible to determine the composition of the unknown mixture. Analysis of the kinetic response may be accomplished by a simple on-line computer enabling direct readout of isotopic assay.”
Traditional neutron detectors that have been used to augment gamma-ray detection systems typically rely on “gross-counting” detect an increased neutron presence that may provide an indication of elevated fission from an unknown source. However, current count-based neutron detectors are generally unable to distinguish neutrons in the environmental background from those emitted by a neutron source. Such gross-counting instruments are especially problematic in situations where the neutron count rate is up to ten times the average background. Count rates above this level are usually readily detectable with counter instruments; count rates below this level, however, pose major problems due to the potential triggering of false alarms.
The radioactive decay of fissile material produces alpha particles which, in turn, produce neutrons through an (α, n) reaction with light elements within the sample. Depending upon the relative amount of unknown source material in a sample, the vast majority of detected neutrons may be from the (α, n) reactions instead of the spontaneous fission of the fissile material. Fission events, however, produce multiple neutrons (e.g., two or three) simultaneously, whereas (α, n) reactions produce neutrons individually and randomly. Coincident counting systems utilize this time distribution difference to distinguish prompt fission neutrons from (α, n) neutrons.
A coincidence counter identifies neutron counts that occur within a short time of each other (i.e., practically simultaneously). In general, coincident detections can be real or accidental in nature. Real coincidences occur when multiple particles are detected that originate from the same reaction, and accidental coincidences occur when two particles are detected in the measurement window, but are not from the same reaction. Particles in these measurements originate primarily from three reactions: spontaneous and induced fission, (α, n) on oxygen, and gamma-ray decay. Accidental coincidence rates are proportional to the size of the measurement gate window. For fast coincidence methods, the coincidence gate is typically kept very short (e.g., 50 ns). Most techniques use a reals plus accidentals (R+A) gate and an accidentals (A) gate and determine the reals rate by the formula R=(R+A)−A.
Some present known coincidence counting techniques define measurement windows that presume that the A window is uncorrelated with the R+A sum. That is, the calculation is based on the assumption that the A gate only contains accidental correlations. Such an assumption is incorrect, since the fission process does not necessarily stop after the end of the A gate, and therefore, real correlations may be rejected simply because they occur during the A gate period.